1. Field of the Invention
The present invention relates to the field of semiconductor wafer processing and, more particularly, to chemical-mechanical polishing of semiconductor wafers.
2. Background of the Related Art
The manufacture of an integrated circuit device requires the formation of various layers (both conductive and non-conductive) above a base substrate to form the necessary components and interconnects. During the manufacturing process, removal of a certain layer or portions of a layer must be achieved in order to pattern and form various components and interconnects. Chemical mechanical polishing (CMP) is being extensively pursued to planarize a surface of a semiconductor wafer, such as a silicon wafer, at various stages of integrated circuit processing. It is also used in flattening optical surfaces, metrology samples, and various metal and semiconductor based substrates.
CMP is a technique in which a chemical slurry is used along with a polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer in combination with the chemical reaction of the slurry disposed between the wafer and the pad, provide the abrasive force with chemical erosion to polish the exposed surface of the wafer (or a layer formed on the wafer), when subjected to a force pressing the wafer to the pad. In the most common method of performing CMP, a substrate is mounted on a polishing head which rotates against a polishing pad placed on a rotating table (see, for example, U.S. Pat. No. 5,329,732). The mechanical force for polishing is derived from the rotating table speed and the downward force on the head. The chemical slurry is constantly transferred under the polishing head. Rotation of the polishing head helps in the slurry delivery as well in averaging the polishing rates across the substrate surface.
A constant problem encountered in CMP processing is that the polishing rate around the periphery (edge) of the substrate is different than that for the interior (center) of the substrate. Various reasons account for this difference. Pad bounce being one cause. The polishing rate difference can also be caused by the variations in the velocity encountered in the rotational movement. The polishing rate may vary depending on the location on the pad where a particular area of the wafer is placed. Some amount of averaging is achieved by rotating the wafer (in some instances, oscillation is also used along with rotation), but polishing rate variations are still noticeable with rotating polishers, such variations resulting in non-uniform polishing across the wafer surface. Thus, an emphasis in CMP processing is to minimize this inequality in polishing rates.
One technique for obtaining a more uniform polishing rate is to utilize a linear polisher. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated for averaging out the local variations, but the global planarity is improved over CMP tools using rotating pads. One such example of a linear polisher is described in a pending application titled "Linear Polisher And Method For Semiconductor Wafer Planarization;" Ser. No. 08/287,658; filed Aug. 9, 1994.
Unlike the hardened table top of a rotating polisher, linear polishers are capable of using flexible belts, upon which the pad is disposed. This flexibility allows the belt to flex and change the pad pressure being exerted on the wafer. The present invention takes this fact into consideration and uses this property to provide for localized pressure variations to be exerted at various locations of the wafer to control the force of the contact of the pad with the wafer in order to obtain a more uniform rate of polish across the wafer.